1. Field of the Invention
The present invention is generally related to cable for transmitting signals, and more particularly related to reduction of crosstalk experienced between the signals.
2. Description of the Related Art
A metal based signal cable for transmitting information across computer networks, generally have a plurality of wire pairs (such as pairs of copper wires) so that a plurality of signals, each signal using a separate wire pair, can be transmitted over the cable at any given time. Having many wire pairs in a cable can have advantages, such as increased data capacity, but as signal frequency used for the signals is increased to also increase data capacity, a disadvantage becomes more evident. As signal frequency increases, the individual signals tend to increasingly interfere with one another due to crosstalk due to the close proximity of the wire pairs. Twisting the two wires of each pair with each other helps considerably to reduce crosstalk, but is not sufficient as signal frequency increases.
Other conventional approaches can be also used to help reduce crosstalk such as using physical spacing within the cable to physically separate and isolate the individual twisted wire pairs from one another to a certain degree. Drawbacks from using additional physical spacing include increasing cable diameter and decreasing cable flexibility.
Another conventional approach is to shield the twisted pairs as represented by the shield twisted pair cable 10 depicted in FIG. 1 as having an internal sheath 12 covered by insulation 14 (such as Mylar), and covered by a conductive shield 16. A drain wire 18 is electrically coupled to the conductive shield 16. The conductive shield 16 can be used to a certain degree to reduce crosstalk by reducing electrostatic and magnetic coupling between twisted wire pairs 20 contained within the internal sheath 12.
An external sheath 22 covers the conductive shield 16 and the drain wire 18. The conductive shield 16 is typically connected to a connector shell (not shown) on each cable end usually through use of the drain wire 18. Connecting the conductive shield 16 to the connector shell can be problematic due to additional complexity of installation, added cable stiffness, special connectors required, and the necessity for an electrical ground available at both ends of the cable 10. Furthermore, improper connection of the conductive shield 16 can reduce or eliminate the effectiveness of the conductive shield and also can raise safety issues due to improper grounding of the drain wire 18. In some improper installations, the conventional continuous shielding of a cable segment is not connected on one or both ends. Unconnected ends of conventional shielding can give rise to undesired resonances related to the un-terminated shield length which enhances undesired external interference and crosstalk at those resonant frequencies
Although conventional approaches have been adequate for reducing crosstalk for signals having lower frequencies, unfortunately, crosstalk remains a problem for signals having higher frequencies.